What One Book Has Changed Your Professional Life? - Second Edition

Greetings,

I'd like to ask you now to respond to a simple, important question:
What's the one book (not blog, not conversation, not Facebook page) that has changed your life asa nonprofit staffer, consultant, volunteer or vendor? Plus the why and how.

Here's an example:
Title: Mission-Based Management by Peter Brinckerhoff
Why/how: Brinckerhoff insists that your organization's mission must permeate every aspect of your work, from technology and program to finances and human resources. It's made a huge difference in my management focus in the seven years since I've read it.